And I'm Audie Cornish. Boeing says its 787 jetliner should resume passenger service early next month. The so-called Dreamliner has been grounded since problems with the lithium ion batteries surfaced back in January. Batteries on two different planes overheated. One of them, a Japan Air Lines jet, caught fire. Boeing is now working with airlines to retrofit its planes with redesigned batteries now housed in fireproof boxes.

The National Transportation Safety Board is investigating the JAL incident, and for the past two days, it has been holding public hearings. NPR's Wendy Kaufman joins us now. And, Wendy, what exactly is the NTSB trying to focus on here?

WENDY KAUFMAN, BYLINE: The focus here, Audie, has not been limited to what went wrong with that JAL battery. Rather, the safety board is trying to look ahead to see what could be done to ensure that this kind of thing just doesn't happen again. In that context, the hearing looked at things like how were the batteries designed, how were they tested, and then how were they approved by federal regulators.

And as you might guess, the toughest questions for Boeing and the regulators were things like: Was the testing rigorous enough? Was it comprehensive enough? How did Boeing come up with these assumptions about how frequently the batteries would fail? And did the FAA rely too much on the data that Boeing gave them and on the tests that Boeing conducted?

CORNISH: Did they actually get any answers to these questions?

KAUFMAN: Well, they got some answers. I think it's fair to say the NTSB wasn't completely satisfied with everything it heard. We should point out here that while this investigation has gone on for more than three months, investigators still don't know the exact cause, the root cause of the battery failure on that plane. They know there was a short circuit in one of the cells, and they know the problem spread to some of the other cells. But they just don't know the precise sequence of events.

CORNISH: Wendy, let's talk some about the testing that was done for the battery. I mean, how rigorous was it?

KAUFMAN: Boeing says it was incredibly rigorous. I think other people might not think it was rigorous enough. Boeing put some 40,000 hours of testing into this battery system. But neither Boeing nor any of its suppliers ever saw a battery fail in the way the batteries failed in January. And that, of course, makes you wonder just what did happen inside those batteries and why wasn't it anticipated or tested for. I mean, that's a really big question that's hanging over all of this.

A senior Boeing official acknowledged yesterday that in hindsight, one of the tests used, a test which involves putting a nail inside a battery to create a short circuit, wasn't, as he put it, conservative enough. This afternoon, the head of the NTSB went beyond that, saying the testing for this 787 battery just wasn't rigorous enough or comprehensive enough.

CORNISH: In the meantime, what role has the FAA had in this?

KAUFMAN: Well, you know, by necessity, and because of a congressional mandate, the FAA has to rely very heavily on manufacturers like Boeing for data and testing. So, for example, while Boeing has a team of battery experts, the FAA just doesn't have that kind of expertise. So it has to rely a lot on designated employees of the company who act on the agency's behalf. So while the board understands there are some limitations, it's concerned that the FAA just wasn't rigorous enough.

One issue was that there was an advisory panel that came out with more rigorous safety testing standards, and the FAA didn't require that Boeing went back and use those new testing standards. It's obvious here, Audie, that technology is changing extremely quickly, coming up with appropriate tests and approval methods. It's very tough and getting tougher.

I think the hope is that coming out of this hearing and out of this inquiry, there are going to be some changes in the processes that people use to design, certify and test in the future.

CORNISH: NPR's Wendy Kaufman speaking to us from the NTSB hearings in Washington. Thank you, Wendy.